IMAGE: NIST's ultrafast electro-optic laser relies on this aluminum "can " to stabilize
and filter the electronic signals, which bounce back and forth inside until fixed
waves emerge at the strongest frequencies and block...
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Credit: D. Carlson/NIST

Physicists at the National Institute of Standards and Technology (NIST) have used common electronics
to build a laser that pulses 100 times more often than conventional ultrafast lasers. The advance could
extend the benefits of ultrafast science to new applications such as imaging of biological materials in
real time.

The technology for making electrooptic
lasers has been around for
five decades, and the idea seems
alluringly simple. But until now
researchers have been unable to
electronically switch light to make
ultrafast pulses and eliminate
electronic noise, or interference.

As described in the Sept. 28 issue of
Science, NIST scientists developed
a filtering method to reduce the heatinduced
interference that otherwise
would ruin the consistency of
electronically synthesized light.

"We tamed the light with an
aluminum can," project leader Scott
Papp said, referring to the "cavity"
in which the electronic signals
are stabilized and filtered. As the
signals bounce back and forth inside
something like a soda can, fixed
waves emerge at the strongest
frequencies and block or filter out other frequencies.

Ultrafast refers to events lasting picoseconds (trillionths of a second) to femtoseconds (quadrillionths of
a second). This is faster than the nanoscale regime, introduced to the cultural lexicon some years ago
with the field of nanotechnology (nanoseconds are billionths of a second).

The conventional source of ultrafast light is an optical frequency comb, a precise "ruler" for light. Combs
are usually made with sophisticated "mode-locked" lasers, which form pulses from many different colors
of light waves that overlap, creating links between optical and microwave frequencies. Interoperation
of optical and microwave signals powers the latest advances in communications, timekeeping and
quantum sensing systems.

"In any ultrafast laser, each pulse lasts for, say, 20 femtoseconds," lead author David Carlson said. "In
mode-locked lasers, the pulses come out every 10 nanoseconds. In our electro-optic laser, the pulses
come out every 100 picoseconds. So that's the speedup here--ultrafast pulses that arrive 100 times
faster or more."

"Chemical and biological imaging is a good example of the applications for this type of laser,"
Papp said. "Probing biological samples with ultrafast pulses provides both imaging and chemical
makeup information. Using our technology, this kind of imaging could happen dramatically faster. So,
hyperspectral imaging that currently takes a minute could happen in real time."

To make the electro-optic laser, NIST researchers start with an infrared continuous-wave laser and
create pulses with an oscillator stabilized by the cavity, which provides the equivalent of a memory
to ensure all the pulses are identical. The laser produces optical pulses at a microwave rate, and
each pulse is directed through a microchip waveguide structure to generate many more colors in the
frequency comb.

The electro-optic laser offers unprecedented speed combined with accuracy and stability that are
comparable to that of a mode-locked laser, Papp said. The laser was constructed using commercial
telecommunications and microwave components, making the system very reliable. The combination
of reliability and accuracy makes electro-optic combs attractive for long-term measurements of optical
clock networks or communications or sensor systems in which data needs to be acquired faster than is
currently possible.

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The research is supported by the Air Force Office of Scientific Research, Defense Advanced Research
Projects Agency, National Aeronautics and Space Administration, NIST and the National Research
Council.

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